A major impediment to the achieving of high luminous efficacy in artificial light sources is the fact that many systems for converting energy into visible light result in the emission of significant quantities of long wavelength infra-red light (to which the eye does not respond) at the expense of visible light of shorter wavelength.
The principal tools available to the developer of light sources have been first to raise the temperature of the radiating body, and second to seek radiating species that have limited emissions in the infra-red. Raising the temperature results in shifting the black-body radiation curve (which sets the upper limit to emission at any wavelength) towards shorter wavelengths, permitting radiating transitions producing visible light to be enhanced. The search for more refractory materials, operable at higher temperatures, has formed the basis for the enhancement of the efficiency of incandescent lamps from the extremely low value of the candle, to the improved gas mantle, to the carbon-filament incandescent lamp, to the present day tungsten-filament lamp. Each in turn was capable of achieving higher operating temperature, and each in turn had higher luminous efficacy, with a smaller and smaller fraction of the energy in the infra-red.
Achieving the excitation of radiating emitting species with few transitions in the infra-red is the basis of the technology of electric discharge lamps, in which the atomic or molecular species excited have only weak emissions into the infra-red, not reaching the blackbody limit, but strong transitions in the shorter wavelength regions of the spectrum.
Despite the clear advantage of tungsten filament incandescent lamps over their predecessors, the radiant emission from these sources is still 90% or more in the infra-red region, not perceived by the eye. Since the development of of the gas-filled tungsten filament incandescent lamp in the second decade of this century, no more-refractory materials capable of higher temperature operation in a light source have been discovered. Despite numerous advances in gas-discharge light sources, the most efficient sources have only a limited number of short wavelength transitions as well, and therefore are either limited in color rendition (low-pressure sodium lamps) or require a phosphor to convert ultraviolet light into visible at considerable loss of efficiency (fluorescent lamps).
It has been the custom to think of the radiative lifetime of an electronically excited state of an atom or molecule as a constant of the universe. However, this is only true when the atom is in free space and able to radiate to infinity with an infinite number of vacuum modes of the electromagnetic field into which to radiate.
Recent research has shown that radiative lifetimes may be in fact strongly modified. The central conclusion of the research, in a variety of configurations, may be called the Cavity Quantum Electrodynamic Principle. Excited states within or coupled to a reflecting cavity or waveguide can only radiate into allowed modes of the cavity or waveguide. In particular if the wavelength of the transition is greater than the cavity cut-off wavelength, the transition probability is zero. (See PHYSICS TODAY January 1989 "Cavity Quantum Electrodynamics" pages 24-30.)
It is well known to the prior art that the radiation from tungsten filament lamps includes only 5-10% of visible light energy, with most of the balance being in the infra-red. It is known to the prior art to operate such filaments for the sake of maximizing the fraction of visible radiation at the highest temperature permitted by the material, as limited by the vaporization of tungsten atoms from the surface. It is well known that as a consequence an inverse relationship holds between efficiency and life of tungsten filament lamps. The higher the efficiency, the shorter is the life.
It is known to the prior art to increase the luminous efficiency of gas flame lanterns by providing a so-called "mantle" in contact with the flame and heated by it to temperatures in the vicinity of 1500.degree. K. The mantles known to the prior art are typically composed of thorium oxide to which a small percentage of cerium oxide has been added. By virtue of having few free electrons, and having a fundamental infra-red absorption/emission band onset at wavelength longer than 5000 nm, the ceramic body of the mantle is a relatively poor radiator of infra-red radiation. The incorporation of cerium adds absorption/emission transitions in the visible part of the spectrum, enhancing the luminous emission at 1500.degree. K. Consequently such so-called "gas mantles" achieve luminous efficacies of 2 lumens/watt or thereabouts at 1500.degree. K, very much more than the 0.2 lumens/watt that could be achieved with a tungsten radiator at that temperature. They are widely used in portable gas-fired lanterns for application where electricity is not available. However, it would be desirable in the construction of such mantles to dispose of the thorium-oxide cerium oxide ceramic body and at the same time increase the efficiency of such mantles.
Accordingly, a principal desirable object of the present invention is to overcome the disadvantages of the prior art.
Another desirable object of the present invention is to provide an energy conversion device which maximizes the conversion of such energy into visible wavelengths.
A still further desirable object of the present invention is to provide an energy conversion device which provides a source of artificial light while minimizing infra-red radiation to the extent that the radiating surface may be operated at a sufficiently lower temperature resulting simultaneously in an increase in efficiency together with an increase in life over incandescent lamps of the prior art.
A desirable object of the present invention is to provide an artificial optical light source which minimizes the emission of infra-red radiation while maximizing emission of visible radiation.
Another desirable object of the present invention is to provide a new and improved optical light source device including an electromagnetic radiation source member and at least one cavity waveguide member.
These and other desirable objects of the invention will in part appear hereinafter and will in part become apparent after consideration of the specification with reference to the accompanying drawings and the claims.